Signal converting systems for use in stereo reproducing systems

ABSTRACT

A signal converter is provided for intermixing first and second audio signals reproduced from a conventional 2-channel recording medium to produce two difference signals which are supplied to a 2 to 4-channel converter for improving the separation between the front and rear signals. The signal converter may be constructed to mix together first and second audio signals at a variable relative amplitude ratio therebetween and with a variable polarity relationship to form two sum signals or two difference signals.

United States Patent Ito et al. 1 May 20, 1975 SIGNAL CONVERTING SYSTEMSFOR USE 3,697,692 10/1912 l-c-ilafler 179/1 00 3,710,023 l/l973 reuzard[N STEREO REPRODUCING SYSTEMS 3,718,773 2/1973 Berkovitz [75] Inventors:Ryosuke Ito; Susumu Takahashl, 3,725,586 4/1973 lida both of Tokyo,Japan 3,745,254 7/1973 Ohta 7 7 91973 Ih'da 179i [73] Assignee: SansuiElectric Co., Ltd., Tokyo, 57 04 l s I I Go Japan PrimaryExaminer-Kathleen H. Claffy [22] Filed: Dec. 18, 1972 AssistantExaminer-Thomas D Amico [211 App] No: 315,928 gnome Agent, or Firm-Hams,Kern, Wallen &

msley [30] Foreign Application Priority Data [57] ABSTRACT Dec. 21, I971Japan 46-[03970 A signal convene], is provided for intermixing first andsecond audio signals reproduced from a conventional [52] US. Cl. 179/1GQ; 179/ 100.4 ST; Lchaune] recording medium to Produce two differ179/100" TD ence signals which are supplied to a 2 to 4-channel Ill.converter f i p i g the separation between the [58] Field of Search"179/1 1 front and rear signals. The signal converter may be 179/1004loo'l 15 BT constructed to mix together first and second audio signalsat a variable relative amplitude ratio therebe- [56] References Citedtween and with a variable polarity relationship to form UNITED STATESPATENTS two sum signals or two difference signals. 3,170,99I 2/1965Glasgal l79/l G 3,329,712 4/1967 Farrell 20 Claims 14 Drawing Figures3,684,835 8/1972 179/! G FL s 0-0" 5 FR m D w o c: 5 3 6 RL 19 0 CD X sE 1m 2 m lo 2i 5 2 RR o+ 9o 1 ECi CONTROL UNIT EC2 SHEET 2 OF 8 CONTROLUNIT FIG. 3

FUENTES P 0 I??? P5020 x522 F5020 XEEE mjmsm mjmsm 0 m m E E E 4 w m 4 RT T- N I G w w 1 F .1 R E T Lm II Mv 1 E5528 zo w N m0 SC RL2+RL3 RR I XE a 9 R I; RR3 R-rL T R rDSCRIMINA 0 E2 Lb g PHA ASE *BECRIMINAT ECiCONTROL UNIT FIG.43

SIGNAL CONVERTING SYSTEMS FOR USE IN STEREO REPRODUCING SYSTEMS Thisinvention relates to a signal converting system which is utilized toreproduce two-channel signals from a conventional two-channel source bya four-channel reproduction system or to reproduce two-channel signalsfrom a matrix four-channel source by a two channel reproduction systemand capable of enhancing the separation between signals in a reproducedsound field.

Recently, a matrix four-channel sound reproduction system has been usedwherein four-channel original signals are converted into two-channelsignals, the twochannel signals are recorded on such recording medium asa phonograph record or a magnetic tape, the two channel signalsreproduced from the recording medium are converted into four-channelsignals corresponding to the original signals and the four-channelsignals are reproduced by four loudspeakers arranged around a listener.

The matrix four-channel reproducing system, however, involves a seriousproblem that the crosstalk between reproducing channels is extremelylarge. Specifcally, in one type of the matrix four-channel reproducingsystems the separation between channels disposed in a diagonal directionis infinity whereas that between adjacent channels equals -3 db.

Although the matrix four-channel reproducing system has beensuccessfully developed as above pointed out, the number of matrixfour-channel stereo records now on the market is far smaller than thatof twochannel stereo records. The matrix four-channel reproducing systemis compatible with conventional twochannel stereo records so that it ispossible to enjoy a four-channel playback of a conventional two-channnelstereo record.

However, owing to the inherently poor separation characteristic of thematrix four-channel reproducing system, when reproducing a conventionaltwo-channel stereo record by a four-channel system, the rear sidelocation of the listening area of a sound image presents a problem. Moreparticularly, where a two-channel stereo record is reproduced by afour-channel system, when only a left signal is reproduced from therecord it is desirable to locate the sound image based on this signal atthe rear-left side of the listening room for the purpose of providing asatisfactory four-channel reproduction. However, with the systemdescribed above, the sound image will be located at an intermediatepoint between the front-left side and the rear left side. This means apoor separation between the front and rear channels.

Matrix four-channel stereo records are also compatible with theconventional two-channel reproducing system. However, since two-channelsignals recorded on a matrix four-channel stereo record usually containfour-channel signals, the separation between the reproduced two-channelsignals is extremely poor.

It is an object of this invention to provide an improved signalconverting system capable of enhancing the separation between thechannels when reproducing sound signals from a conventional two-channelstereo recording medium by a four-channel reproduction system or from amatrix four-channel recording medium by a two-channel reproductionsystem.

According to one aspect of this invention there is provided a signalconverting system for reproducing two-channel stereo signals from asource of twochannel signals by means of a four-channel stereoreproducing system. said signal converting system comprising acombination of first signal converting means including means convertedto receive the first and second channel signals from the two-channelsignal source for combining the first channel signal with a 180outof-phase portion of the second channel signal and means for combiningthe second channel signal with a 180 out-of-phase portion of the firstchannel signal; and second signal converting means connected to receivetwo output signals from the first signal converting means for convertingthe two output signals into fourchannel signals.

According to another aspect of this invention there is provided a signalconverting system for reproducing two-channel stereo signals from asource of matrix four-channel signals by means of a two-channel stereoreproducing system, said signal converting system comprising means forcombining the first channel signal with the second channel signal at avariable relative amplitude ratio therebetween and with a variablepolarity relationship and means for combining the second channel signalwith the first channel signal at a variable relative amplitude ratio andwith a variable polarity relationship.

The present invention can be more fully understood from the followingdetailed description when taken in connection with reference to theaccompanying drawings, in which:

FIG. 1 shows a block diagram of a signal converting system embodying theinvention;

FIG. 2 shows a connectiondiagram of a matrix circuit shown in FIG. 1;

FIG. 3 shows a block diagram of a portion of a modified embodiment ofthis invention;

FIG. 4 shows a modification of the embodiment shown in FIG. 3',

FIGS. 5, 6 and 7 show connection diagrams of different signal convertersutilized in this invention;

FIG. 8 shows a connection diagram of one example of a control unitutilized in the embodiment shown in FIGS. 3 and 4',

FIG. 9 shows a connection diagram of one example of a variable matrixcircuit utilized in the embodiments shown in FIGS. 3 and 4;

FIG. I0 is a graph showing the output characteristic of the control unitshown in FIG. 8;

FIG. 11 is a simplified block diagram of still another embodiment of theinvention;

FIG. 12 shows a block diagram of another variable matrix circuit;

FIG. 13 shows a block diagram of another variable matrix circuit; and

FIG. 14 shows a block diagram of a modification of the variable matrixcircuit of FIG. 13.

With reference now to FIG. 1 of the accompanying drawing illustrating apreferred embodiment of this invention, reference numeral 10 designatesa suitable two-channel source which may be a conventional stereophonograph record, a stereo recorded magnetic tape or an FM stereoreceiver. The first and second audio signals L and R produced by thetwo-channel source 10 are applied to a matrix circuit 12 via a signalconverter 11 to be described later. The matrix circuit 12 may beconstructed as shown in H0. 2, for example. The circuit shown in FIG. 2is constructed to convert the first and second audio signals L and Rinto fourchannel signals consisting of FL (front-left), FR (frontright),RL (rear-left) and RR (rear-right), which are expressed by the followingequations:

where each A represents a matrix coefficient having a value of about0.4. In the four-channel reproducing system, it is usual to install fourloudspeakers SFL, SFR, SRL and SRR about a listener 13 in a listeningroom 14.

The output FL from matrix circuit 12 is applied to the correspondingloudspeaker SFL through a phase shifter 15 and a power amplifier 16while the output FR to loudspeaker SFR through a phase shifter 17 and apower amplifier 18. Similarly, the outputs RL and RR are supplied tocorresponding loudspeakers SRL and SRR respectively through phaseshifters 19 and 21 and power amplifiers 20 and '22. The purpose of thephase shifters l5, l7, l9 and 21 is to maintain front signals FL and FRat the in-phase relationship throughout the entire range of audiofrequencies and to bring the rear signals RL and RR into in-phaserelationship which have been 180 out-of-phase.

Where only a left signal is impressed upon the matrix circuit 12, bothsignals FL and RL are designated by L. Accordingly, under theseconditions, although it is desirable to locate the sound image of thisleft signal L at the position of the loudspeaker SRL, actually the soundimage is located at a mid-point between the loudspeakers SFL and SRL. Aswill be described later, the signal converter 11 is constructed to formdifference signals L' (LXaR) and R (RXaL) in response to the left andright signals. Accordingly, responsive to the signals L-aR and R-aL, thematrix circuit 12 operates to form the following signals:

Accordingly, where only the left signal L is impressed upon signalconverter 11, the outputs FL and RL from the matrix circuit 12 are shownby L( l-Aa) and L( 1+Aa), respectively. This means that the sound imagecorresponding to the left signal L is located at a position closer tothe loudspeaker SRL.

One example of the signal converter 11 shown in FIG. 5 is provided withinput terminals 21 and 22 connected to receive the left and rightsignals L and R, respectively, and a pair of output terminals 23 and 24.The first input terminal 21 is connected to the input terminal of afirst inverter 25, and a first potentiometer resistor 26 is connectedbetween the output terminal of the first inverter and the first inputterminal 21. The sliding arm 27 of the first potentiometer resistor 26is connected to the second input terminal 22 through serially connectedresistors 28 and 29, and the junction between these resistors 28 and 29is connected to the second output terminal 24. The second input terminal22 is connected to the input terminal of a second inverter 30, and asecond potentiometer resistor 31 is connected between the outputterminal of the second inverter and the second input terminal 22. Thesliding arm 32 of the second potentiometer resistor 31 is connected tothe first input terminal 21 through serially connected resistors 33 and34, the junction therebetween being connected to the first outputterminal 23. The sliding arms 27 and 32 of the first and secondpotentiometer resistors 26 and 31 are mechanically interlocked as shownby dotted lines.

When the sliding arms 27 and 32 are positioned at the centers of firstand second potentiometers 26 and 31 the left and right signals L and Rare produced at the first and second output terminals 23 and 24,respectively.

When the sliding arms 27 and 32 are moved in the direction of arrows 0along the potentiometer resistors 26 and 31, respectively, there arerespectively derived from the output terminals 23 and 24 two differencesignals LaR annd R-aL each having varying relative amplitude ratiobetween the signals L and R, whereas when the sliding arms 27 and 32 aremoved in the direction of arrows b there are obtained two sum signalsL-i-BR and R+BL each having varying relative amplitude ratio. In theembodiment shown in FIG. I it is advantageous to use the potentiometerresistors 26 and 31 such that their sliding arms are positioned to theright of their mid-points to produce the two difference signals.

In a modified signal converter shown in FIG. 6, each of thecollector-emitter paths of first and second transistors Q, and O isconnected across a source indicated by +8 and the ground, and the baseelectrodes of these transistors are connected to input terminals 21 and22 respectively through coupling capacitors. In parallel with thecollector-emitter paths of the transistors Q, and Q, are connected firstand second potentiometer resistors 36 and 37 provided with sliding arms38 and 39, respectively. The sliding arm 38 of the first potentiometerresistor 36 is connected to the emitter electrode of transistor Qthrough serially connected resistors 40 and 41, whereas the sliding arm39 of the other potentiometer resistor 37 is connected to the emitterelectrode of transistor Q, through serially connected resistors 42 and43. Junctions between resistors 42 and 43 and between 40 and 41 areconnected to output terminals 23 and 24, respectively. The sliding arms38 and 39 of two potentiometer resistors 36 and 37 are mechanicallyinterlocked each other as shown by dotted lines.

ln this embodiment, the collector resistors 44 and 45 and the emitterresistors 46 and 47 of transistors Q and Q are made to have an equalvalue. Again, when sliding arms 38 and 39 are moved in the direction ofdotted arrows at two difference signals L-aR and R-aL are produced atthe output terminals 23 and 24 respectively whereas when these slidingarms are moved in the direction of solid line arrows b two sum signalsL-t-BR and R-l-BL are produced.

In another embodiment of the signal converter shown in FIG. 7, each ofthe collector-emitter paths of transistors Q and Q is connected acrossthe source. The base electrodes of these transistors are connected toinput terminals 21 and 22 respectively through coupling capacitors whilethe collector electrodes are connected to output terminals 23 and 24respectively. The emitter electrodes of transistors Q and Q, areinterconnected through a resistor R. In this signal converter twodifference signals each having a predetermined fixed amplitude ratiobetween the signals L and R are derived out from output terminals 23 and24.

A modified embodiment of this invention shown in FIG. 3 comprises avariable matrix circuit 48 and a control unit 49. This modificationillustrates a decoder capable of reproducing with satisfactory channelseparation sound signals recorded on a matrix four-channel recordingmedium by a four-channel system. In such a decoder the phaserelationship between the twochannel signals, for example, L=LF+AFR+RL+jARR and R=FR+AFL-jRR-jARL which are reproduced from the matrixfour-channel recording medium is detected by the control unit 49constituted by a phase discriminator or a level comparator, and thematrix coefficients of the matrix circuit 48 are controlled by theoutputs ECl and EC2 from the control unit 49. When the two-channelsignals reproduced from a conventional two-channel recording medium areapplied to such a decoder system the control unit 49 can not control thematrix circuit 48 because the two-channel signals are generally inphase. For this reason, in such a case, it is desirable to providesignal converter 11 as shown in FIGS. 5, 6 or 7 on the input side of thecontrol unit 49 so that when only one signal is reproduced from thetwo-channel recording medium, the input signals to the control unit 49will have opposite phases thereby enabling the control unit 49 tocontrol the variable matrix circuit 48. For example, where only L signalpresents, it is possible to locate the sound image of the signal at theposition of loudspeaker SRL by the operation of the variable matrixcircuit 48, this improving the separation between the front channels andthe rear channels.

To aid the understanding of the invention, the constructions andoperations of variable matrix circuit 48 and control unit 49 will bedescribed briefly hereunder.

FIG. 8 shows a circuit diagram of a phase discriminator which comprisesa first limiter 50 including transistors 51 and 52 connected to receivethe L signal and a second limiter 53 including transistors 54 and 55connected to receive the R signal. The first and second limiters 50 and53 have large amplification gains and operate to transform the signals Land R into rectangular wave signals. Two output signals of oppositepolarities produced by the second limiter 53 are amplified by first andsecond amplifiers 56 and 58 including transistors 57 and 59respectively. The outputs from the first and second amplifiers 56 and 58are supplied to a first switching circuit 60 and a second switchingcircuit 61 respectively including bridge connected diodes D, to D, anddiodes D to D,,, thereby causing these switching circuits ON and OFFalternately. The output from the first limiter 50 is coupled to thecommon input of the first and second switching circuits 60 and 61, whilethe output terminals of these switching circuits 60 and 61 are groundedthrough capacitors 62 and 63 respectively, and are connected to a pointof reference voltage (in this case, +B/2 volts) through potentiometers64 and 65, respectively. The slidable arms of the potentiometers 64 and6S supply the first and second control outputs EC 1 and EC2.

The phase discriminator constructed as above described operates toswitch the left signal L by alternately rendering ON and OFF the firstand second switching circuits 60 and 61 in response to the right signalR thereby discriminating the phase difference between the right and leftsignals R and L. FIG. shows the operating characteristic of the phasediscriminator showing that the first and second control outputs EC 1 andEC2 vary symmetrically but in opposite directions about the referencelevel, which is equal to about +B/2 volts in the phase discriminatorshown in FIG. 8.

FIG. 9 illustrates an example of the variable matrix circuit 48 whereina first matrix circuit associated with the front channels comprises afirst differential amplifier 91 including transistors 92 and 93. Theleft signal L is coupled to the base electrode of transistor 92 whilethe right signal R is coupled to the base electrode of transistor 93through an inverter 94 including a transistor 95. The collectorelectrode of transistor 92 is connected to the first output terminal ofthe matrix circuit while the collector electrode of transistor 93 isconnected to the second output terminal of the matrix circuit through aninverter 96 comprising a transistor 97. A first control circuit 99including a field effect transistor 100 is capacitively connected inparallel with a common emitter resistor 98 of transistors 92 and 93which constitute the differential amplifier 91. The gate electrode ofthe field effect transistor 100 is connected to a control input terminalso that it acts as a variable resistor. The first control circuit 99operates to vary the AC impedance of the emitter circuits of transistors92 and 93 in accordance with the magnitude of the control input EC] soas to control the common mode gain of the differential amplifier 91.

The second matrix circuit associated with the rear channels comprises asecond differential amplifier 106 including transistors 107 and 108. Theleft signal L is coupled to the base electrode of transistor 107,whereas the right signal R is coupled to the base electrode oftransistor 108. The collector electrodes of transistors 107 and 108 arerespectively connected to the third and fourth output terminals of thematrix circuit. A second control circuit including a field effecttransistor 111 is capacitively connected in parallel with a commonemitter resistor 109 for transistors 107 and 108. The gate electrode offield effect transistor 111 is connected to a control input terminal.The second control circuit 110 operates in the same manner as the firstcontrol circuit 99 so as to control the common mode gain of the seconddifferential amplifier in accordance with the magnitude of the controlinput EC2.

The operation of the variable matrix circuit shown in FIG. 9 will bebriefly described as follows: Where the composite signals L' and R aresubstantially in phase, the control input ECl is large and the controlinput EC2 is small. Consequently, the AC impedance of the emittercircuits of transistors 92 and 93 is decreased whereby the gain of thefirst differential amplifier 91 is increased, whereas that of the seconddifferential amplifier 106 is decreased. increase in the gain of thefirst differential amplifier 91 results in the increase in the level ofthe left signal L which is derived out from the collector electrode oftransistor 92 and in the decrease in the level of the right signal Rcontributing to increasing the cross-talk. On the other hand, the levelof the right signal R derived out from the collector electrode oftransistor 93 is increased and the level of the left signal Lcontributing to increasing the cross-talk is decreased. Accordingly, theseparation between the front channel is improved with the increase inthe signal level. In the rear channels, as the gain of the seconddifferential amplifier 106 decreases, the separation degrades with thedecrease in the signal level.

in FIG. 3, when the signal converter 11 produces two difference signalsL-ozR and R-aL in in-phase relationship, the outputs ECl and EC2 of thecontrol unit 49 have a large level and a small level, respectively.Then, the gains of the first and second differential amplifiers 91 and106 are respectively increased and decreased, and the first output FLand second output FR of the first matrix circuit 90 are formed mostly ofsignal L and signal R, respectively. Thus, signal L is localized at theloudspeaker SFL, and signal R at the loudspeaker SFR.

If supplied with only signal L or signal R, the signal converter 11produces two output signals L and AL or two output signals R and AR,both in reverse-phase relationship. As a result, the outputs ECl and EC2of the control unit 49 have a small level and a large level,respectively, thereby to decrease the gain of the first differentialamplifier 91 and to increase the gain of the second differentialamplifier 106. Consequently, the outputs RL and RR of the second matrixcircuit 105 come to be filled mostly with signal L and signal R,respectively. That is, if only signal L is supplied to the converter 11,it is localized at the loudspeaker SRL. Similarly, if only signal R issupplied to the converter 11, it is localized at the loudspeaker SRR.

To generalize the above, signals to be localized somewhere to the leftand to the right of midway between the left and right loudspeakers for2-channel reproduction are localized at the loudspeaker SFL and at theloudspeaker SFR, respectively. Signals to be localized at the left andright loudspeakers are localized at the loudspeakers SRL and SRR,respectively.

When the signal converter 11 produces two sumsignals L+BR and R+BL, theoutputs ECl and EC2 of the control unit 49 have a large level and asmall level, respectively. As a result, the in-phase components in theinput signals L and R are localized in front of the reproduction soundfield, and the reverse-phase components, e.g. revibration components, ofthese signals are localized at the back of the sound reproduction field.

The detail of the construction and operation of the variable matrixcircuit 48 and the control unit 49 and their modifications are fullydescribed in the copending US Pat. application Ser. No. 298,933, filedOct. 19, 1971, of the title Decoder for use in 4-2-4 matrix playbacksystem now US. Pat. No. 3825684.

FIG. 4 shows a modification of the circuit shown in FIG. 3.

In this modification, the variable matrix circuit 48 and control unit 49are connected to receive the two channel signals L and R via the signalconverter 11. When the two channel signals L and R are in-phase andinclude crosstalk components therebetween, the differ ence signalsproduced by the signal converter 11 are caused to be decreased in levelas well as enhanced in separation. Accordingly, the arrangement of FIG.4 in which the separation previously enhanced two-channel signals aresupplied to the variable matrix circuit 48 can provide somewhat betterseparation characteristics than the arrangement of FIG. 3.

FIG. 12 is a block diagram of a variable matrix circuit according toanother embodiment used in the arrangement of FIG. 4. With referencefirst to the front channels, there are provided a first matrix circuit130 adapted to produce sum signals (L'+R) and -(L'+R') of oppositepolarities from the composite signals L' and R' produced by the signalconverter 11, and a second matrix circuit 13] adapted to produce adifference signal (L'R). The difference signal (L'R) is applied to athird matrix circuit 133 via a variable gain amplifier 132 to be addedtherein to the outputs from the first matrix circuit 130. The firstvariable gain amplifier 132 is controlled by the first control outputECI from the control unit 49 and has an amplification gain f whichvaries from 0 to 2.41 with resepct to the gain of the first matrixcircuit 130. The third matrix circuit I23 functions to produce a firstoutput expressed by 1+1) L'+ l-f) R and a second output expressed by lj)L l-l-f) R which is phase inverted by an inverter 134.

Associated with the rear channels there are provided a fourth matrixcircuit 135 adapted to produce difference signals (L'R') and L'R') ofthe opposite polarities and a fifth matrix circuit 136 adapted toproduce a sum signal (L'+R). The sum signal (L'+R') is applied through asecond variable gain amplifier 137 to a sixth matrix circuit 138 whereit is added to the outputs (L'-R) and (L'R') from the fourth matrixcircuit 135.

The second variable gain amplifier 137 has an amplification gain b whichvaries from 0 to 2.41 with respect to the gain of the fourth matrixcircuit 135. Accordingly, the sixth matrix circuit 138 produces a thirdoutput expressed by (1+b) L( 1-b) R and a fourth output expressed by(1+b) R'-( lb)L'. The gains of the first and second variable gainamplifiers 132 and 137 are varied in the opposite directions by thecontrol outputs ECl and ECZ from the control unit 49.

FIG. 13 shows a block diagram of another embodiment of the variablematrix circuit. FIG. 13 is different from FIG. 12 in that it isincorporated with the following circuit components. More particularly,there are provided a 0 phase shifter and a 45 phase shifter 171 whichintroduce a phase difference of 45 between the composite signal L andthe composite signal R. Responsive to the outputs from phase shifters170 and 171 an adder 172 provides an output (L'+R' +45), whereas asubtractor 173 provides an output (LR' +45). A phase discriminator 174for controlling the left and right channels operates to detect the phasedifference between the output signals (=L'+lR') and RL3 (=L'lR').Similarly, a matrix circuit 177 is connected to receive the signal Lthrough a variable gain amplifier 178, and the signal R to produceoutputs PR3 (=R'+rL') and RR3 (=R'rL') where l and r represent the gainsof the variable gain amplifiers 176 and 178 respectively. These gainsare controlled in the opposite directions in a range of from 0 to 3.414by the outputs Er and El from the phase discriminator 174. The gains ofthe variable gain amplifiers 132 and 137 are controlled in the oppositedirections in a range of from 0 to 3.4l4 by the outputs Ef and Eb ofphase discriminator as the control unit 49 for controlling rear andfront channels by detecting the phase difference between the signals L'and R. The output ELl from the matrix circuit 133 is coupled to oneinput of an adder 179 via a l/ 2 attenuator I80 and the output FL3 fromthe matrix circuit is applied to the other input of adder 179.Similarly, the output FRI from the matrix circuit 133 is applied to oneinput of an adder 181 through a 1/ V7 attenuator 182 whereas the outputPR3 from the matrix circuit 177 is coupled to the other input of adder181. Likewise, the output RLl of matrix circuit 138 is applied to oneinput of an adder 183 through a 1/ W2 attenuator 184 while the outputRL3 of matrix circuit 175 is applied to the other input of adder 183.The output RRl of matrix circuit I38 is coupled to one input of adder185 through a l 2 attenuator 186 and the output RR3 of the matrixcircuit 177 is applied to the other input'of adder 185.

it will be clear that the four channel signals FL. FR, RL and RR areexpressed by the following equations.

F 1G. 14 shows a block diagram of a modification of the variable matrixcircuit shown in FIG. 13. in FIG. 14, variable gain amplifiers 132 and137 are controlled respectively by the outputs Ef and Eb from acomparator as the first control unit 49 for the front-rear control whichdetects the difference in the levels of the sum signal (L'+R) and thedifference signal (L'R'), and variable gain amplifiers 176 and 178 arecontrolled respectively by the outputs El and Er from a comparator 190as the second control unit for the left and right control which detectsthe difference in the levels of the left signakL' and the right signalR. In this manner, it is possible to obtain the same effect as thevariable matrix circuit shown in FIG. 13 by detecting the difference inthe signal levels.

FIG. 11 shows another embodiment of this invention in which a signalconverter is used for the two-channel playback of a matrix four-channelsource. The first and second audio signals L and R reproduced from thematrix four-channel source which may be a matrix fourchannel stereorecord, a matrix four-channel magnetic tape or a matrix four-channel FMstereo signal source are expressed by the following equations:

R=FR+AFLjRR-jARL Where only the rear-left signal RL is present, L=RL andR=ARL. Where signal converter 11 shown in FIG. 5 or 6\is aadjusted toprovide two sum signals L'=L+BR and R'=R+BL which are applied to aconventional two-channel reproducer 122, the signal converter describedabove will produce outputs L'=RL-ABRL and R'=ARL+BRL. Accordingly,outputs L'=( lA RL and R= can be produced by a proper adjustment of B.This means that it is possible to make infinity the separation betweeenrear signals. Thus, it will be seen that incorporation of a signalconverter which produces sum signals into a combination of a matrixfour-channel source containing rear signals of an especially large leveland a two-channel reproducing system is advantageous in the reproductionof a matrix four-channel sound source by a two-channel system.

Where only a front left signal FL presents, L=FL and R=AFL. when thepotentiometer resistors of the signal converter shown in FIGS. and 6 areadjusted to provide difference signals L'=L-aR and R'=RaL respectively,the outputs will become L'=FL-AaFL and R'=AFLaFL. Thus, it is possibleto obtain outputs L'"-( l-A) FL and R'=0 by a proper adjustment of a.This shows that it is possible to make infinity the separation betweenfront signals. Accordingly, when a signal converter 11 as shown in FIG.5 or 6 is used in the circuit shown in FIG. 11 it is possible to controlthe separation between rear signals or front signals in accordance withthe content recorded on a matrix fourchannel recording medium. In thecase of a jazz music. the level of the rear signal is relatively highwhereas in the case of classical music the level of the rear signal islow.

What we claim is:

1. A signal converting system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a four-channel stereo reproducing system, said systemcomprising a combination of:

first signal converting means connected to receive the first and secondchannel signals for producing first and second composite signals bycombining the first and second channel signals;

a control unit responsive to the phase relationship between the firstand second composite signals for producing first and second controloutputs, the levels of which vary in opposite relationship; and

second signal converting means connected to receive the first and secondchannel signals for producing four-channel output signals by combiningthe first and second channel signals, said second signal convertingmeans including means for controlling at least a relative amplituderatio between the first and second channel signals contained in each ofa pair of output signals in accordance with the level of said firstcontrol output and means for controlling at least a relative amplituderatio between the first and second channel signals contained in each ofanother pair of output signals in accordance with the level of saidsecond control output.

2. A signal converting system according to claim 1 wherein the firstcomposite signal is a sum signal of the first and second channel signalswhich contains the first channel signal at a larger amplitude level andthe second channel signal at a smaller amplitude level, and the secondcomposite signal is a sum signal of the first and second channel signalswhich contains the first channel signal at a smaller amplitude level andthe second channel signal at a larger amplitude level.

3. A signal converting system according to claim 1 wherein the firstcomposite signal is a difference signal of the first and second channelsignals which contains the first channel signal at a larger amplitudelevel and the second channel signal at a smaller amplitude level, andthe second composite signal is a difference signal of the first andsecond channel signals which contains the first channel signal at asmaller amplitude level and the second channel signal at a largeramplitude level.

4. A signal converting system according to claim 1 wherein said secondsignal converting means comprises:

a first differential amplifier having first and second output terminalsderiving a pair of output signals and first and second input terminals,said first input terminal being connected to receive the first channelsignal;

phase reversing means for reversing the phase of the second channelsignal;

means for coupling the output of said phase reversing means to saidsecond input terminal of said first differential amplifier;

means for controlling the gain of said first differential amplifier inaccordance with the level of said first control output;

a second differential amplifier having first and second output terminalsderiving another pair of output signals and first and second inputterminals connected to receive said first and second channel signalsrespectively; and

means for controlling the gain of said second differential amplifier inaccordance with the level of said control output.

5. A signal converting system according to claim 1 wherein said secondsignal converting means comprises:

means connected to receive the first and second channel signals L and Rfor producing a first output signal substantially proportional tol+f)L+( lj)R where f represents a first variable matrix coefficient;

means connected to receive the first and second channel signals L and Rfor producing a second output signal substantially proportional to(1+j)R+(lflL;

means connected to receive the first and second channel signals L and Rfor producing a third output signal substantially proportional to(l+b)L-(- l-b) R where b represents a second variable matrixcoefficient;

means connected to receive the first and second channel signals L and Rfor producing a third output signal substantially proportional to (l+b)Rmeans for varying the first variable matrix coefficient f in accordancewith the level of said first control output; and

means for varying the second variable matrix coetficient b in accordancewith the level of said second control output.

6. A signal converting system according to claim 1 which furthercomprises a further control unit for producing third and fourth controloutputs, the levels of which vary in opposite relationship in responseto the first and second composite signals, and

wherein said second signal converting means comprises:

means connected to receive the first and second channel signals L and Rfor producing a first output signal substantially proportional to (l+f+{ilLH l-f-l- /51 )L; wherefand l represent variable matrix coefficients;

means connected to receive the first and second channel signals L and Rfor producing a second output signal substantially proportional to (1+f+/)R+( lf+ 1/ 2 r)L, where r represents a variable matrix coefficient;

means connected to receive the first and second chaanel signals L and Rfor producing a third output signal substantially proportional to (l+b+{2 )L( lb+ film, where b represents a variable matrix coefficient;

means connected to receive the first and second channel signals L and Rfor producing a fourth out- 12 put signal substantially (l+b+ 4'2')R-1b+ 421.; means for varying the coefficient f in accordance with thelevel of said first control output means for varying the coefficient bin accordance with the level of said second control output;

means for varying the coefficient 1 in accordance with the level of saidthird control output; and

means for varying the coefficient r in accordance with the level of saidfourth control output.

7. A signal converting system according to claim I wherein said controlunit comprises a phase discriminator for detecting the phase differencebetween the first and second composite signals.

8. A signal converting system according to claim 1 wherein said controlunit comprises a level comparator for comparing levels of the sum anddifference signals of the first and second composite signals.

9. A signal converting system according to claim 6 wherein said furthercontrol unit comprises a phase discriminator for detecting phasedifference between the sum and difference signals of the first andsecond composite signals.

10. A signal converting system according to claim 6 wherein said furthercontrol unit comprises a level comparator for comparing the levels ofthe first and second composite signals.

11. A signal converting system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a fourchannel stereo reproducing system, said system comprisinga combination of:

first signal converting means connected to receive the first and secondchannel signals for producing first and second composite signals bycombining the first and second channel signals;

a control unit responsive to the phase relationship between the firstand second composite signals for producing first and second controloutputs, the lev els of which vary in opposite relationship; and

second signal converting means connected to receive the first and secondcomposite signals for producing four-channel output signals by combiningthe first and second composite signals, said second signal convertingmeans including means for controlling at least a relative amplituderatio between the first and second composite signals contained in eachof a pair of output signals in accordance with the level of said firstcontrol output and means for controlling at least a relative amplituderatio between the first and second composite signals contained in eachof another pair of output signals in accordance with the level of saidsecond control output.

12. A signal converting system according to claim 11 wherein the firstcomposite signal is a sum signal of the first and second channel signalswhich contains the first channel signal at a larger amplitude level andthe second channel signal at a smaller amplitude level, and the secondcomposite signal is a sum signal of the first and second channel signalswhich contains the first channel signal at a smaller amplitude level andthe second channel signal at a larger amplitude level.

13. A signal converting system according to claim ll wherein the firstcomposite signal is a difference signal of the first and second channelsignals which contains the first channel signal at a larger amplitudelevel and proportional to the second channel signal at a smalleramplitude level, and the second composite signal is a difference signalof the first and second channel signals which contains the first channelsignal at a smaller amplitude level and the second channel signal at alarger amplitude level.

14. A signal converting system according to claim 11 wherein said secondsignal converting means comprises:

a first differential amplifier having first and second output terminalsderiving a pair of output signals and first and second input terminals,said first input terminal being connected to receive the first compositesignal;

phase reversing means for reversing the phase of the second compositesignal;

means for coupling the output of said phase reversing means to saidsecond input terminal of said first different'ial amplifier;

means for controlling the gain of said first differential amplifier inaccordance with the level of said first control output;

a second differential amplifier having first and second output terminalsderiving another pair of output signals and first and second inputterminals connected to receive said first and second composite signalsrespectively; and

means for controlling the gain of said second differential amplifier inaccordance with the level of said second control output.

15. A signal converting system according to claim 11 wherein said secondsignal converting means comprises:

means connected to receive the first and second composite signals L' andR for producing a first output signal substantially proportional to(l+j)L-H-( lf)R' where f represents a first variable matrix coefficient;

means connected to receive the first and second composite signals L andR for producing a second output signal substantially proportional to(1+f)R'+( l-f)L';

means connected to receive the first and second composite signals L andR for producing a third output signal substantially proportional to(l+b)L'-( lb)R where b represents a second variable matrix coefficient;

means connected to receive the first and second composite signals L andR for producing a third output signal substantially proportional to(l+b)R'-( lb)L;

means for varying the first variable matrix coefficient f in accordancewith the level of said first control output; and

means for varying the second variable matrix coefficient b in accordancewith the level of said second control output.

16. A signal converting system according to claim 11 which furthercomprises a further control unit for producing third and fourth controloutputs, the levels of which vary in opposite relationship in responseto the first and second composite signals, and

wherein said second signal converting means comprises: means connectedto receive the first and second composite signals L and R for producinga first output signal substantially proportional to (l+f+ {2 L'+( l-f+450R. where fand I represent variable matrix coefficients; meansconnected to receive the first and second composite signals L and R' forproducing a second output signal substantially proportional to (l+f+fi)R'+( lf+ EHL'. where r represents a variable matrix coefficient;

means connected to receive the first and second composite signals L andR' for producing a third output si nal substantially proportional to(H-b+ 2)L(lb+ {27)R', where h represents a variable matrix coefficient;

means connected to receive the first and second composite signals L andR for producing a fourth output signal substantially proportional to(l+b+ 47 R'( lb+ Jim;

means for varying the coefficient f in accordance with the level of saidfirst control output;

means for varying the coefficient b in accordance with the level of saidsecond control output; means for varying the coefficient l in accordancewith the level of said third control output; and means for varying thecoefficient r in accordance with the level of said fourth controloutput.

17. A signal converting system according to claim 11 wherein saidcontrol unit comprises a phase discriminator for detecting the phasedifference between the first and second composite signals.

18. A signal converting system according to claim 11 wherein saidcontrol unit comprises a level comparator for comparing levels of thesum and difference signals of the first and second composite signals.

19. A signal converting system according to claim 16 wherein saidfurther control unit comprises a phase discriminator for detecting phasedifference between the sum and difference signals of the first andsecond composite signals.

20. A signal converting system according to claim 16 wherein saidfurther control unit comprises a level comparator for comparing thelevels of the first and second composite signals.

1. A signal converting system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a four-channel stereo reproducing system, said systemcomprising a combination of: first signal converting means connected toreceive the first and second channel signals for producing first andsecond composite signals by combining the first and second channelsignals; a control unit responsive to the phase relationship between thefirst and second composite signals for producing first and secondcontrol outputs, the levels of which vary in opposite relationship; andsecond signal converting means connected to receive the first and secondchannel signaLs for producing four-channel output signals by combiningthe first and second channel signals, said second signal convertingmeans including means for controlling at least a relative amplituderatio between the first and second channel signals contained in each ofa pair of output signals in accordance with the level of said firstcontrol output and means for controlling at least a relative amplituderatio between the first and second channel signals contained in each ofanother pair of output signals in accordance with the level of saidsecond control output.
 2. A signal converting system according to claim1 wherein the first composite signal is a sum signal of the first andsecond channel signals which contains the first channel signal at alarger amplitude level and the second channel signal at a smalleramplitude level, and the second composite signal is a sum signal of thefirst and second channel signals which contains the first channel signalat a smaller amplitude level and the second channel signal at a largeramplitude level.
 3. A signal converting system according to claim 1wherein the first composite signal is a difference signal of the firstand second channel signals which contains the first channel signal at alarger amplitude level and the second channel signal at a smalleramplitude level, and the second composite signal is a difference signalof the first and second channel signals which contains the first channelsignal at a smaller amplitude level and the second channel signal at alarger amplitude level.
 4. A signal converting system according to claim1 wherein said second signal converting means comprises: a firstdifferential amplifier having first and second output terminals derivinga pair of output signals and first and second input terminals, saidfirst input terminal being connected to receive the first channelsignal; phase reversing means for reversing the phase of the secondchannel signal; means for coupling the output of said phase reversingmeans to said second input terminal of said first differentialamplifier; means for controlling the gain of said first differentialamplifier in accordance with the level of said first control output; asecond differential amplifier having first and second output terminalsderiving another pair of output signals and first and second inputterminals connected to receive said first and second channel signalsrespectively; and means for controlling the gain of said seconddifferential amplifier in accordance with the level of said controloutput.
 5. A signal converting system according to claim 1 wherein saidsecond signal converting means comprises: means connected to receive thefirst and second channel signals L and R for producing a first outputsignal substantially proportional to (1+f)L+(1-f)R where f represents afirst variable matrix coefficient; means connected to receive the firstand second channel signals L and R for producing a second output signalsubstantially proportional to (1+f)R+ (1-f)L; means connected to receivethe first and second channel signals L and R for producing a thirdoutput signal substantially proportional to (1+b)L-(1-b) R where brepresents a second variable matrix coefficient; means connected toreceive the first and second channel signals L and R for producing athird output signal substantially proportional to (1+b)R- (1-b)L; meansfor varying the first variable matrix coefficient f in accordance withthe level of said first control output; and means for varying the secondvariable matrix coefficient b in accordance with the level of saidsecond control output.
 6. A signal converting system according to claim1 which further comprises a further control unit for producing third andfourth control outputs, the levels of whicH vary in oppositerelationship in response to the first and second composite signals, andwherein said second signal converting means comprises: means connectedto receive the first and second channel signals L and R for producing afirst output signal substantially proportional to (1+f+ Square Root 2)L+(1-f+ Square Root 2l )L; where f and l represent variable matrixcoefficients; means connected to receive the first and second channelsignals L and R for producing a second output signal substantiallyproportional to (1+f+ Square Root 2)R+ (1-f+ Square Root 2r)L, where rrepresents a variable matrix coefficient; means connected to receive thefirst and second chaanel signals L and R for producing a third outputsignal substantially proportional to (1+b+ Square Root 2)L- (1-b+ SquareRoot 2l)R, where b represents a variable matrix coefficient; meansconnected to receive the first and second channel signals L and R forproducing a fourth output signal substantially proportional to (1+b+Square Root 2)R-(1-b+ Square Root 2rL; means for varying the coefficientf in accordance with the level of said first control output means forvarying the coefficient b in accordance with the level of said secondcontrol output; means for varying the coefficient l in accordance withthe level of said third control output; and means for varying thecoefficient r in accordance with the level of said fourth controloutput.
 7. A signal converting system according to claim 1 wherein saidcontrol unit comprises a phase discriminator for detecting the phasedifference between the first and second composite signals.
 8. A signalconverting system according to claim 1 wherein said control unitcomprises a level comparator for comparing levels of the sum anddifference signals of the first and second composite signals.
 9. Asignal converting system according to claim 6 wherein said furthercontrol unit comprises a phase discriminator for detecting phasedifference between the sum and difference signals of the first andsecond composite signals.
 10. A signal converting system according toclaim 6 wherein said further control unit comprises a level comparatorfor comparing the levels of the first and second composite signals. 11.A signal converting system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a four-channel stereo reproducing system, said systemcomprising a combination of: first signal converting means connected toreceive the first and second channel signals for producing first andsecond composite signals by combining the first and second channelsignals; a control unit responsive to the phase relationship between thefirst and second composite signals for producing first and secondcontrol outputs, the levels of which vary in opposite relationship; andsecond signal converting means connected to receive the first and secondcomposite signals for producing four-channel output signals by combiningthe first and second composite signals, said second signal convertingmeans including means for controlling at least a relative amplituderatio between the first and second composite signals contained in eachof a pair of output signals in accordance with the level of said firstcontrol output and means for controlling at least a relative amplituderatio between the first and second composite signals contained in eachof another pair of output signals in accordance with the level of saidsecond control output.
 12. A signal converting system according to claim11 wherein the first composite signal is a sum signal of the first andsecond channel signals which contains the first channel signal at alarger amplitude level and the second channel signal at a smalleramplitude level, and the second composite signal is a sum signal of thefirst and second channel signals which contains the first channel signalat a smaller amplitude level and the second channel signal at a largeramplitude level.
 13. A signal converting system according to claim 11wherein the first composite signal is a difference signal of the firstand second channel signals which contains the first channel signal at alarger amplitude level and the second channel signal at a smalleramplitude level, and the second composite signal is a difference signalof the first and second channel signals which contains the first channelsignal at a smaller amplitude level and the second channel signal at alarger amplitude level.
 14. A signal converting system according toclaim 11 wherein said second signal converting means comprises: a firstdifferential amplifier having first and second output terminals derivinga pair of output signals and first and second input terminals, saidfirst input terminal being connected to receive the first compositesignal; phase reversing means for reversing the phase of the secondcomposite signal; means for coupling the output of said phase reversingmeans to said second input terminal of said first differentialamplifier; means for controlling the gain of said first differentialamplifier in accordance with the level of said first control output; asecond differential amplifier having first and second output terminalsderiving another pair of output signals and first and second inputterminals connected to receive said first and second composite signalsrespectively; and means for controlling the gain of said seconddifferential amplifier in accordance with the level of said secondcontrol output.
 15. A signal converting system according to claim 11wherein said second signal converting means comprises: means connectedto receive the first and second composite signals L'' and R'' forproducing a first output signal substantially proportional to (1+f)L+ +(1-f)R'' where f represents a first variable matrix coefficient; meansconnected to receive the first and second composite signals L'' and R''for producing a second output signal substantially proportional to(1+f)R'' +(1-f)L''; means connected to receive the first and secondcomposite signals L'' and R'' for producing a third output signalsubstantially proportional to (1+b)L'' -(1-b)R'' where b represents asecond variable matrix coefficient; means connected to receive the firstand second composite signals L'' and R'' for producing a third outputsignal substantially proportional to (1+b)R''-(1-b)L'' ; means forvarying the first variable matrix coefficient f in accordance with thelevel of said first control output; and means for varying the secondvariable matrix coefficient b in accordance with the level of saidsecond control output.
 16. A signal converting system according to claim11 which further comprises a further control unit for producing thirdand fourth control outputs, the levels of which vary in oppositerelationship in response to the first and second composite signals, andwherein said second signal converting means comprises: means connectedto receive the first and second composite signals L'' and R'' forproducing a first output signal substantially proportional to (1+f+Square Root 2)L''+(1-f+ Square Root 2l)R'', where f and l representvariable matrix coefficients; means connected to receive the first andsecond composite signals L'' and R'' for producing a second outputsignal substantially proportional to (1+f+ Square Root 2)R'' +(1-f+Square Root 2r)L'', where r represents A variable matrix coefficient;means connected to receive the first and second composite signals L''and R'' for producing a third output signal substantially proportionalto (1+b+ Square Root 2)L''-(1-b+ Square Root 2l)R'', where b representsa variable matrix coefficient; means connected to receive the first andsecond composite signals L'' and R'' for producing a fourth outputsignal substantially proportional to (1+b+ Square Root 2)R'' -(1-b+Square Root 2r)L''; means for varying the coefficient f in accordancewith the level of said first control output; means for varying thecoefficient b in accordance with the level of said second controloutput; means for varying the coefficient l in accordance with the levelof said third control output; and means for varying the coefficient r inaccordance with the level of said fourth control output.
 17. A signalconverting system according to claim 11 wherein said control unitcomprises a phase discriminator for detecting the phase differencebetween the first and second composite signals.
 18. A signal convertingsystem according to claim 11 wherein said control unit comprises a levelcomparator for comparing levels of the sum and difference signals of thefirst and second composite signals.
 19. A signal converting systemaccording to claim 16 wherein said further control unit comprises aphase discriminator for detecting phase difference between the sum anddifference signals of the first and second composite signals.
 20. Asignal converting system according to claim 16 wherein said furthercontrol unit comprises a level comparator for comparing the levels ofthe first and second composite signals.